In general, transformers include electric devices that transfer electric energy from one circuit to another circuit (or multiple circuits) to increase (i.e., “step-up”) or decrease (i.e., “step-down”) voltage. The transfer of energy may be accomplished through electromagnetic mutual induction, i.e., where time-varying current through a primary conductor produces a time-varying magnetic flux through a secondary conductor. As a result of Faraday's law of induction, the changing flux induces an electromotive force in the secondary conductor that gives rise to a current. The voltage in the secondary conductor is typically provided by the ratio of the number of windings of the secondary conductor relative to the number of windings in the primary conductor multiplied by the voltage of the primary conductor—where this ratio is often referred to as a “turns ratio.” In general, if the turns ratio of secondary to primary is greater than one, the result is a step-up transformer, and if the turns ratio of secondary to primary is less than one, the result is a step-down transformer.
A balanced-to-unbalanced transformer, which is also referred to in the art and herein as a “balun” transformer, is a device used for matching an unbalanced line to a balanced load. A common type of a balun transformer is a flux-coupled balun transformer, which is created by winding two separate wires around a magnetic core, and grounding one side of the primary winding. This creates an unbalanced condition on the primary side, and a balanced condition on the secondary side. In addition, the secondary side can have an arbitrary ratio of turns relative to the primary side (i.e., the turns ratio of n:1), creating an impedance ratio. The flux-coupled balun transformer will induce an alternating current (AC) voltage in the secondary of n times the voltage in the primary, while the current will be n times smaller than in the primary, giving an output impedance of n2, where n is the ratio of turns in the secondary to turns in the primary.
Thus, balun transformers may be used to change impedance levels between stages while maintaining direct current (DC) isolation between the stages of a differential circuit. Balun transformers may also or instead be used in transmitters, where they can provide signal isolation between local oscillators and radio frequency (RF) and intermediate frequency (IF) sections of a balanced upconverter, or coupling output stages of a push-pull power amplifier. Other applications for balun transformers may include discriminators, phase detectors, antenna feeds, and the like.
As stated above, the inductors of a transformer, such as a balun transformer, may be wound around a core, directly impacting the mutual inductance between the primary and secondary inductors, and therefore the performance of the transformer. Balun transformers may be formed by placing primary and secondary windings within metal layers of a substrate (e.g., a gallium arsenide substrate)—e.g., the windings may be formed by placing planar metal traces in the substrate. It may be advantageous to provide a substantially symmetrical balun transformer with as few metal layers as possible, as this can reduce manufacturing complexity, size, and cost. Further, a high degree of electrical symmetry may help maintain circuit isolation, and provide improved broadband frequency response. However, certain geometries give rise to substantial inter-winding capacitance, which can limit the operating bandwidth of a device. Also, the center tap in balun transformers is often disposed at an undesirable location, resulting in an asymmetric geometry, the addition of metal layers, or additional direct current (DC) voltage loss due to higher resistance. There remains a need for improved balun transformers.